A compilation of Washington Post stories that used a definition of four people shot as a “mass shooting.”

Wednesday’s massacre in San Bernardino was the latest mass shooting in America. By some counts — including a heavily cited version — it was the 354th of the year. By a narrower definition, it was the 40th. In light of the headlines about mass shootings, there’s been a growing debate about what should count.

Definitions for statistics are far from perfect. We must assign numbers to complex events. We must group and categorize people and scenarios that are anything but generic. And we must be aware of the imperfections we place in data by making these decisions.

In order to do anything about guns — to identify a problem, to uncover trends, to have productive debates, to prevent more tragedies — one must draw the lines somewhere.

This tool, based on shootings tracked by Reddit users, demonstrates a few of the ways “mass shooting” can be defined. The data only includes shootings where at least four people were shot, but it’s among the most thorough sources of gun violence in America. Experiment with your own definition for “mass shooting,” and see how many have happened so far this year.

You may find that no satisfying definition exists.

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